Reland "[flutter_tools] Fix DTD "Stream already subscribed" error and wait for analysis over DTD in widget preview" - #193658

… wait for analysis over DTD in widget preview" (flutter#193658)

## Description

Relands flutter#192930 (which was
reverted in flutter#193637).

### What changed since the original PR
- In `AnalysisServer`, store and cancel the `stdout` and `stderr` stream
subscriptions in `dispose()` before closing `_analyzingController` and
`_errorsController`, and guard `_analyzingController.add` with
`!_analyzingController.isClosed` in `_handleProgress`. Previously,
`$/progress` notifications arriving from the language server during or
after `AnalysisServer.dispose()` threw `Bad state: Cannot add new events
after calling close` in `widget_preview_scaffold_test.dart`.
- In `widget_preview_test_helpers.dart`, await `analysisServer.onExit`
after `analysisServer.dispose()` in `addTearDown` so the analysis server
process has fully exited before temporary directory cleanup on Windows.
- Added a hermetic unit test in `analyze_continuously_test.dart`
verifying that `AnalysisServer` does not throw when `$/progress`
notifications arrive during or after `dispose()`.

### Original description
During widget preview startup or file updates:
1. Concurrent calls to retrieve previews or wait for analysis (e.g.,
initial command startup racing with file watcher events) could
concurrently invoke `_waitForLspService()`, causing duplicate
`streamListen` calls to the Dart Tooling Daemon. DTD throws JSON-RPC
error 103 (`Stream already subscribed`) when a client attempts to
subscribe to a stream it is already listening to.
2. `WidgetPreviewDtdServices` previously considered the `Lsp` service
ready as soon as *any* `Lsp` method was registered on DTD (or upon
receiving the first `ServiceRegistered` event for `Lsp`). Because the
analysis server registers its LSP methods sequentially before posting an
`initialized` event on the `Lsp` stream, calls to
`Lsp.dart/workspace/analysis/complete` or
`Lsp.dart/workspace/getFlutterWidgetPreviews` could race ahead of their
registration and fail with `-32601 (method not found)`.
3. When connecting to a pre-existing analysis server over DTD
(`--dtd-url`), `LspPreviewDetector.analysisServer` is `null`, causing
`analysisServer?.waitForAnalysis()` calls during startup and file
updates to be no-ops.

This change:
- Deduplicates in-flight calls to `_waitForLspService()` using a shared
future (`_waitForLspServiceFuture`) so concurrent callers await the same
LSP registration check instead of issuing duplicate DTD requests.
- Separates `lspServiceAvailable` (whether an analysis server has begun
registering `Lsp` services on DTD) from `_allLspServicesRegistered`
(whether all required widget preview LSP methods in
`kRequiredLspServices` are registered), and updates
`_waitForLspServiceHelper()` to listen for the `initialized` event on
the `Lsp` stream before checking `getRegisteredServices()`.
- Consolidates LSP DTD calls into `_callLspService()` and removes the
`kMethodNotFound` polling loop now that `_waitForLspService()` ensures
all required LSP methods are registered.
- Safely catches and ignores `RpcErrorCodes.kStreamAlreadySubscribed` on
`streamListen` calls in `WidgetPreviewDtdServices` (`safeStreamListen`).
- Adds `WidgetPreviewDtdServices.waitForAnalysis()` (invoking
`Lsp.dart/workspace/analysis/complete` over DTD), unifies
`LspPreviewDetector.waitForAnalysis()` so widget preview waits for
analysis completion on both tool-managed and pre-existing analysis
servers, and moves the startup `waitForAnalysis()` call inside the DTD
error-handling `try / catch` block in `WidgetPreviewStartCommand`.
- Adds hermetic regression tests covering concurrent/pre-subscribed DTD
streams, partial vs. full LSP service registration (`Lsp.initialized`),
and waiting for analysis over DTD.

Fixes flutter#192424

## Pre-launch Checklist

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responsibilities, or I am not using AI tools.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant in-code documentation (doc comments with
`///`).
- [x] If this PR introduces a new feature or capability, I created and
linked a website documentation issue or PR in [flutter/website] (or
verified none is needed).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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